The present invention relates generally to methods and systems of screening particles, and particularly, such methods that employ Raman scattering for separating particles of different types.
Raman spectroscopy is a powerful technique that allows identifying molecules via their characteristic spectral fingerprints. It relies on inelastic scattering of incident photons by a molecule, via coupling to its vibrational modes, to provide an essentially unique signature for that molecule. In particular, such inelastic scattering (commonly known as Raman scattering) can cause a decrease or an increase in the scattered photon energy, which appear as “Stokes” and “anti-Stokes” peaks in a wavelength-dispersed spectrum of the scattered photons. A drawback of Raman spectroscopy is that the probability for occurrence of such scattering is small (typically presented as the scattering cross-section).
Raman scattering cross sections can, however, be significantly enhanced by placing the molecules on or near roughened nanoscale metal surfaces. Such a mode of performing Raman spectroscopy is commonly known as surface enhanced Raman spectroscopy (SERS). It has also been demonstrated that significant enhancement in SERS cross-sections can be achieved by placing Raman-active molecules on both aggregates of, and single metallic nanoparticles. However, different metallic nanoparticles can provide widely varying enhancement factors. In other words, not all metallic nanoparticles are equally SERS-active.
Accordingly, there is a need for methods and systems for screening metallic nanoparticles for their SERS activity, and there is a need for such methods that can efficiently separate SERS-active nanoparticles from SERS-inactive nanoparticles.